1. Field of the Invention
The present invention relates to a new method for manufacturing a printed circuit board for electronic devices and an electronic device for which the printed circuit board is used.
2. Description of the Related Art
The surface mounting technology is known as a method for manufacturing a printed circuit board for electronic devices. In the surface mounting technology, electronic parts can be mounted densely, and therefore, it provides a merit of realizing miniaturization and weight-reduction of electronic devices.
According to a printed circuit board manufacturing (assembling) process by the surface mounting technology, a solder paste is first printed through a metal mask on a printed circuit board which is an object to which electronic parts are connected by soldering (in the present invention, a printed circuit board which is an object to which electronic parts are connected by soldering is referred to as “a substrate”), the electronic parts are mounted on the printed solder paste, and then, the parts are connected to the substrate by heating to a temperature not less than the melting point of the solder component.
A solder paste is a creamy material composed of a mixture of a solder component and a flux for paste. When heated to a temperature not less than the melting point of the solder component, the solder component which is usually in the shape of a powder is melted to be a unified form and forms a solder connection part. A metal mask is a metal plate having through openings corresponding to positions of electrodes on the substrate. By using the metal mask, it is possible to supply and apply a solder paste to the substrate electrodes.
Conventionally, lead (Pb) has been used for the solder paste. However, in view of the consideration on the environmental pollution, the need for a lead-free solder has become stronger in recent years. For example, in EU, the RoHS Directive will be enforced as of July, 2006. Accordingly, use of lead (Pb) will be prohibited in the general electronic devices, making application of a lead-free solder an indispensable element in assembling of printed circuit boards.
As a lead-free solder, a Sn-3Ag-0.5Cu solder (melting point 218° C.) is being developed for application to various products. However, since it has a melting point that is 35° C. higher than the conventional Sn-37Pb solder (melting point 183° C.), there is a problem that, if there is an electronic part with a low heat resistance, application to a product with the part is not possible due to the thermal damage incurred to the part.
A Sn-57Bi-1Ag solder (melting point 138° C.) is another system which is a low melting point, lead-free, Sn-58Bi solder (melting point 139° C.) with 1 wt. % of Ag added. By the addition of Ag, the fatigue life is greatly improved due to its finer metal structure. It is known that when the amount of Ag to be added exceeds 1 wt. %, the fatigue life is shortened due to crystallization of coarse crystals of Ag3Sn, an intermetallic compound (Japanese Patent No. 3347512 (paragraph [0012]).
However, the application of a Sn-57Bi-1Ag solder has been limited until now. The reason is that there are some conventional electronic parts that have connection terminals with Sn-10Pb plating, and they cannot provide secure connection reliability to the solder connection parts due to a Bi—Pb—Sn alloy with a melting point not more than 100° C. generated at the solder connection parts during the connection by soldering.
Regarding electronic parts for the surface mounting technology, there is an LSI package called BGA (Ball Grid Array)/CSP (Chip Scale Package) having solder balls at the bottom surfaces of the electronic part bodies. The material for the solder balls has a composition of Sn-3Ag-0.5Cu. In this case, when a Sn-57Bi-1Ag composition is used for a solder paste, for example, the Ag concentration exceeds 1 wt. % at the solder connection parts, caused by the transfer of Ag in the Sn-3Ag-0.5Cu composition during the connection by soldering, leading to a shorter fatigue life.
On the other hand, when a QFP (Quad Flat-leaded Package) part having connection terminals without Ag in their metal composition is mounted with a Sn-58Bi solder paste, a high connection reliability cannot be realized since no Ag is included in the solder connection parts.
Thus, when assembling of a printed circuit board is carried out only with either one of the Sn-58Bi solder paste and Sn-57Bi-1Ag solder paste, there will be solder connection parts partly generated with decreased connection reliability, resulting in decrease in reliability of the whole device.